Plant Tissues

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Cross-section of a sunflower stem — ©Berkshire Community College Bioscience Image Library flickr.com CC BY 2.0
Diagram of the three tissue systems in a sunflower stem (<span lang="la" class="biological-name">Helianthus</span> sp.)
In this preparation (stained with safranin and astranin), the three main tissue systems are clearly visible: **dermal** (epidermis), **ground** (cortex and pith parenchyma), and **vascular** (xylem, stained red, and phloem).

The plant organism, like an animal’s, is not a chaotic accumulation of cells but consists of ordered groups united by a common structure and function. These groups are called tissues.

In modern botany, a tissue is understood as an evolutionarily established, stable system of cells that share a common origin, similar structure, and perform one or more common functions (Evert, 2006; Serebryakova et al., 2006). The cells of a tissue are physically connected to each other, often share common cell walls, and are linked by cytoplasmic bridges — plasmodesmata — which enable their collective work (Raven et al., 2013; Serebryakova et al., 2006).

Tissues arose during evolution as a response to two major challenges that plants faced when colonising land: the need to withstand gravity without the support of water, and the need to regulate water loss in the aerial environment. Before colonising land, the bodies of algae (even multicellular ones) were built from a few similar cell types that were hardly specialised. Only in the largest seaweeds (e.g., brown algae) do rudimentary tissues resembling conducting and mechanical elements appear (Serebryakova et al., 2006). However, the major leap in structural complexity occurred precisely with the emergence of vascular plants (Mauseth, 2017).

The colonisation of land by plants (beginning about 450 million years ago) required a radical reorganisation of their body plan. The earliest land plants (rhyniophytes) lacked true roots and leaves; their bodies were dichotomously branching axes — telomes. Yet they already possessed the first dermal tissues with stomata for protection against desiccation and gas exchange, as well as vascular tissues (protoxylem and protophloem) for transporting water and assimilates (Serebryakova et al., 2006). Later, during evolution, leaves formed from telomes (Zimmermann’s telome theory), and the appearance of secondary lateral meristems (cambium) allowed the formation of perennial woody forms that reach enormous sizes. Thus, the emergence of tissues was a key evolutionary acquisition that made the dominance of plants on land possible.

1. Function and Purpose (Agronomic Context)

Why do plants need a tissue system?

If the unicellular alga Chlamydomonas spends its entire life in water, where nutrients, gases, and waste products diffuse across the entire cell surface, a land plant faces a different set of challenges. It must: 1) resist desiccation in the air, 2) lift its photosynthetic organs toward the light against gravity, 3) transport water from the soil to heights of 100 m or more, and 4) distribute organic substances from leaves to roots, flowers, and storage organs.

Performing these diverse tasks is impossible with a single cell type. Therefore, a multicellular plant is a complex “factory” where different groups of cells (tissues) specialise in different stages of life support. Such a division of labour is the main reason for the existence of tissues (Evert, 2006).

Division of labour allows the plant to:

Use resources efficiently: build “roads” (vascular tissues) only where transport is needed, and “walls” (mechanical tissues) only where support is needed.
Protect vulnerable zones (meristems) from external influences using dermal tissues.
Create depots for storing substances (in ground tissues) during dormancy or to fuel initial growth.

Basic functions of tissues

Each tissue system contributes to crop yield and plant health.

Growth (meristems). Meristems provide indeterminate growth (unlimited increase in mass) — a property that humans exploit in agriculture: after mowing grass or removing the tip of a tomato plant, lateral and intercalary meristems produce new shoots, and the cambium annually forms new layers of wood (Mauseth, 2017). By managing meristem activity (e.g., through pinching, topping), the agronomist regulates the balance between vegetative and generative mass.
Protection (dermal tissues). The epidermis with its cuticle and waxy bloom prevents excessive transpiration, which is critically important for drought tolerance of varieties. The periderm (cork) protects trunks from pests and pathogens. In agronomy, it is important to preserve the integrity of dermal tissues when applying pesticides (it is through the cuticle and stomata that these agents penetrate) (Mauseth, 2017; Raven et al., 2013).
Transport (vascular tissues).
- Xylem (wood) conducts water and mineral salts from the roots to the leaves (upward flow). The speed and efficiency of this process determine how much water the plant can evaporate and, consequently, how much carbon dioxide it can assimilate for photosynthesis. Infection of the xylem by fungi (Fusarium, Verticillium) causes vascular wilt — one of the most dangerous diseases of agricultural crops.
- Phloem (bast) transports assimilates (sugars, amino acids) from donor leaves to acceptors (roots, fruits, storage organs). Disruption of phloem transport (e.g., by girdling or infection with phytoplasmas) leads to starvation of the root system and reduced yield. The rate of phloem transport depends on the balance of boron and sugars, which is important for foliar fertilisation (Evert, 2006; Raven et al., 2013).
Support (mechanical tissues). Collenchyma (living, plastic) supports growing organs, while sclerenchyma (dead, lignified) supports mature parts, including cereal stems and wood. Mechanical tissues prevent lodging of grain crops and breakage of branches under the weight of fruit. Breeding for strong straw is one of the most important traits of high‑yielding wheat and rice varieties (Mauseth, 2017; Serebryakova et al., 2006).
Photosynthesis and storage (ground tissues). Chlorenchyma (parenchyma with chloroplasts) is the main site of photosynthesis. Its thickness and structure affect crop productivity. Storage parenchyma (in tubers, bulbs, seeds, root crops) is what humans grow most agricultural crops for (potatoes, sugar beet, cereals). Understanding the structure of storage tissues is directly related to product quality and shelf life.
Secretion and chemical defence (secretory tissues). Resin ducts, laticifers, glandular hairs, and idioblasts produce substances that make the plant inedible to pests (alkaloids, glycosides, tannins) or, conversely, attract pollinators (nectar‑producing tissues). Many valuable secondary metabolites (rubber, resins, essential oils, medicinal alkaloids) accumulate in secretory structures (Evert, 2006; Serebryakova et al., 2006). For agronomy, the role of secretory tissues in wound healing (e.g., gum exudation in fruit trees) is also important.

Thus, knowledge of the functions of each tissue allows not only the diagnosis of diseases and nutrient deficiencies but also targeted manipulation of the production process — from variety selection to fertiliser and plant protection strategies.

2. Composition and General Organisation of the Tissue Level

Difference between tissue, organ, and cell. Simple and complex tissues

It is important to distinguish three levels of organisation in the plant body:

Cell — the smallest structural and functional unit of life, possessing its own protoplast, nucleus and organelles.
Tissue — a group of similar cells united to perform a specific task (e.g., conducting water).
Organ — a part of the body that has a definite shape and consists of several different tissues working together (e.g., a leaf consists of dermal, photosynthetic and vascular tissues).

In terms of composition, two types of tissues are distinguished:

Simple tissues consist of one cell type (e.g., parenchyma consists only of parenchyma cells). This is typical for ground and mechanical tissues.
Complex tissues consist of two or more cell types that perform different but coordinated functions (e.g., xylem includes conducting elements, fibres and parenchyma cells). (Evert, 2006; Serebryakova et al., 2006)

Hierarchy: cell → tissue → organ → organ system → organism

In plants, unlike animals, we do not distinguish organ systems in the classical sense (since plants lack, for example, a circulatory or nervous system). Instead, we recognise tissue systems that run throughout the body and unite the organs. Traditionally (after Sachs), three tissue systems are distinguished: dermal, vascular, and ground (storage/assimilation). These systems are already established in the embryo and maintain continuity from roots to leaves (Raven et al., 2013; Evert, 2006). The concept of “organ system” is less applicable to plants, but one can speak of the shoot system (stems, leaves, buds) and the root system, each of which is built from the three aforementioned tissue systems (Serebryakova et al., 2006).

2.1. Tissue systems

Diagram of leaf structure — ©Zephyris commons.wikimedia.org CC BY-SA 3.0
Detailed structure of a leaf
The diagram shows the arrangement of tissues in a leaf: dermal tissue (upper and lower epidermis with cuticle and stomata), ground tissue (palisade and spongy mesophyll), and elements of vascular tissue (veins). Key plant cell organelles are also indicated.

In the body of a vascular plant, tissues are not scattered randomly but form three continuous tissue systems that can be traced from roots to leaves. This concept, dating back to the work of Julius Sachs (19th century), reveals the unity of structure of all organs (Raven et al., 2013; Evert, 2006).

Dermal system
- In the primary body, it is represented by the epidermis — a single layer of cells covering leaves, young stems and roots (where it is called the rhizodermis).
- During secondary thickening, the epidermis is replaced by periderm (cork), which consists of three components: cork cambium (phellogen), cork (phellem), and phelloderm (Evert, 2006; Mauseth, 2017).
- Function — a barrier between the plant’s internal environment and the outside world (protection against desiccation, overheating, pathogens, mechanical damage). Gas exchange (stomata) and water uptake by the root occur precisely through the dermal system.
Vascular system
- Consists of two complex tissues: xylem (wood) and phloem (bast).
- Xylem conducts water and dissolved mineral salts from the roots to the leaves (upward flow), while phloem conducts assimilates (primarily sucrose) from the leaves to sites of use or storage (downward flow).
- These tissues originate from procambium (primary) and cambium (secondary) and form a continuous network that permeates all organs. In the stem of dicotyledons, the vascular system is usually organised as a cylinder (ring of bundles), in the root as a central vascular cylinder (stele), and in the leaf as veins (Raven et al., 2013; Serebryakova et al., 2006).
Ground system
- Fills the space between the dermal and vascular systems. It is composed mainly of parenchyma cells, but may also include collenchyma and sclerenchyma.
- In the stem, a distinction is made between the cortex (located between the epidermis and the vascular cylinder) and the pith (in the centre of the stem). In the leaf, the ground tissue forms the mesophyll, where photosynthesis takes place. In the root, the ground tissue constitutes the primary cortex, lying between the rhizodermis and the central cylinder.
- Functions — photosynthesis, storage, gas exchange (aerenchyma), and in some plants, mechanical support.

2.2. Cellular basis

The diversity of tissues is determined by three main characteristics of cells: shape, cell wall structure, and the state of the protoplast (Evert, 2006; Mauseth, 2017).

Cell shape:

Parenchyma cells — isodiametric (roughly equal in all dimensions) or slightly elongated. Typical of ground tissues where compact arrangement and contact are required.
Prosenchyma cells — strongly elongated (tens or hundreds of times longer than wide), with pointed ends. This is the shape of fibres, conducting elements, and some sclereids. Elongation provides the function of long‑distance conduction and mechanical strength.

Type of cell wall:

Primary wall — thin, elastic, capable of stretching as the cell grows. Composed of cellulose microfibrils embedded in a matrix of hemicelluloses and pectins. Characteristic of living cells: parenchyma, collenchyma, meristems.
Secondary wall — thick, rigid, often lignified. Deposited on the inner surface of the primary wall after cell growth ceases. It contains almost no pectins but much cellulose and lignin. Characteristic of dead conducting and mechanical cells (vessels, tracheids, fibres, sclereids).
Cutinisation — impregnation of walls with cutin (a waxy substance) — a property of epidermal cells, especially the outer walls. Cutin makes the wall impermeable to water and gases.
Suberisation — impregnation of walls with suberin, making them waterproof and leading to the death of the protoplast (cork cells, endodermis, exodermis) (Serebryakova et al., 2006).

Presence of protoplast, vacuoles, plastids:

Living cells (parenchyma, collenchyma, cambium, phloem) contain a parietal layer of cytoplasm with a nucleus, organelles, and a large central vacuole. They may contain chloroplasts (chlorenchyma) or leucoplasts (storage parenchyma).
Dead cells (xylem, sclerenchyma) lose their protoplast upon maturation; their cavity is filled with water or air. Vacuoles and plastids are absent.

2.3. Intercellular space

Cells within a tissue are not always tightly packed. Gaps between cell walls form intercellular spaces (intercellulars). They arise either by separation of cells along the middle lamella (schizogenously) or by the dissolution of whole cells (lysigenously) (Evert, 2006).

Aerenchyma — parenchyma with very large intercellular spaces forming a system of air channels (characteristic of aquatic and wetland plants, e.g., water lily, rice). It serves to ventilate tissues experiencing oxygen deficiency (roots in waterlogged soil) and also provides buoyancy.
Secretory cavities — intercellular spaces around which secretory cells are located, releasing essential oils, resins, or gums into the cavity. Examples: oil glands in citrus peel (lysigenous), resin ducts in conifer wood (schizogenous) (Serebryakova et al., 2006).
Stomatal pores — specialised intercellular spaces formed by a pair of guard cells. They serve for gas exchange and transpiration.

2.4. Plasticity

One of the fundamental properties of plant tissues is plasticity, i.e., the ability to change their structure and even cell type in response to external conditions or damage. This is because many living cells (especially parenchyma and collenchyma) retain the ability to dedifferentiate and return to meristematic activity (Evert, 2006; Serebryakova et al., 2006).

Examples of plasticity:
Upon wounding (cut, frost damage, insect attack), adjacent living parenchyma cells become activated, start dividing, and form a wound meristem, which then creates a protective cork — callus.
When switching from vegetative growth to flowering (induction of floral genes), the shoot meristem changes the type of tissues it produces — instead of leaves, it forms sepals, petals, etc. This is morphogenetic plasticity.
In aquatic plants, when submerged, aerenchyma forms, and when emerged, normal parenchyma forms. This is ecological plasticity of tissues.

Tissue plasticity is widely used in agronomy: cuttings, budding, grafting, and clonal micropropagation are based on the ability of parenchyma cells and cambium to regenerate (Mauseth, 2017).

3. Classification, Types, Diversity

The diversity of plant tissues can be systematised according to two main principles: by origin (ontogenetic) and by function (physiological‑anatomical). Both approaches are important for understanding how tissues arise during plant development and what role they play in the plant’s life (Evert, 2006; Serebryakova et al., 2006).

3.1. Principles of classification

By origin: primary and secondary tissues

Depending on which meristem the tissue is derived from, we distinguish:

Primary tissues — formed from primary meristems (apical, intercalary). They make up the body of herbaceous plants (herbs) and young organs of trees. Primary tissues include: epidermis, primary cortex, primary xylem and phloem (from procambium), and pith.
Secondary tissues — arise from secondary lateral meristems (cambium and phellogen). They are characteristic of perennial woody plants and constitute the bulk of trunks, branches, and old roots. Secondary tissues include: secondary xylem (wood) and phloem (bast), and periderm (cork) (Mauseth, 2017; Raven et al., 2013).

By functional‑morphological principle

This principle underlies most textbooks. It distinguishes six main groups of tissues according to their role in the plant. Each of these groups will be discussed in detail in separate articles of our project; here we give only a brief overview for navigation.

3.2. Types of tissues

Meristems (formative tissues)

Main page: Meristems (formative tissues)

Tissues that retain the ability to divide throughout the life of the plant. They provide unlimited growth (indeterminism). They are subdivided into:

Apical meristems — located at the tips of roots and shoots, providing growth in length.
Intercalary meristems — located at the bases of internodes in grasses, enabling rapid stem growth “from below”.
Lateral meristems — cambium and phellogen, providing growth in thickness.
Wound (traumatic) meristems — arise from living parenchyma cells upon injury, form callus and heal wounds (Evert, 2006; Mauseth, 2017).

Dermal tissues

Main page: Dermal tissues

Protect the plant from adverse environmental influences, excessive evaporation, and pathogen entry. Regulate gas exchange and transpiration. Include:

Epidermis (cuticle) — primary dermal tissue of leaves, young stems, and roots (rhizodermis). Often covered with a cuticle and wax, contains stomata and trichomes (hairs).
Periderm (cork) — secondary dermal tissue that replaces the epidermis on ageing stems and roots. Consists of cork cambium (phellogen), cork (phellem), and phelloderm.
Ritidome (bark) — tertiary dermal complex formed by the initiation of several successive periderms. Characteristic of old tree trunks (Raven et al., 2013; Serebryakova et al., 2006).

Vascular tissues

Main page: Vascular tissues

Provide long‑distance transport of substances throughout the plant. Form a continuous branched network connecting all organs.

Xylem (wood) — conducts water and mineral salts from the roots to the leaves (upward flow). Main elements: tracheids and vessel elements (dead cells with lignified walls). Also contains xylem parenchyma and fibres.
Phloem (bast) — transports organic substances (assimilates) from leaves to roots, fruits, buds (downward flow). Main elements: sieve tubes (in angiosperms) or sieve cells (in gymnosperms) — living but enucleate cells, as well as companion cells and bast fibres (Evert, 2006; Mauseth, 2017).

Mechanical tissues (supporting tissues, sclerenchyma)

Main page: Mechanical tissues

Give the plant strength and resistance to static and dynamic loads (wind, fruit weight, snow). Their strength is based on thickened and often lignified cell walls.

Collenchyma — living tissue composed of cells with unevenly thickened primary walls. Located in young, growing organs (under the epidermis of stems and petioles). Provides plastic support without hindering growth.
Sclerenchyma — dead tissue with evenly thickened, lignified secondary walls. Divided into fibres (long, flexible, forming the basis of wood and bast) and sclereids (stone cells, giving hardness to nut shells, fruit stones, seed coats) (Serebryakova et al., 2006; Mauseth, 2017).

Ground tissues (parenchyma)

Main page: Ground (parenchymatous) tissues

The most abundant and functionally diverse group, filling the spaces between other tissues. Cells are living, with thin primary walls.

Assimilation parenchyma (chlorenchyma) — contains chloroplasts, performs photosynthesis (main tissue of the leaf — mesophyll).
Storage parenchyma — accumulates starch, proteins, oils, water (tubers, bulbs, cotyledons, endosperm, stem pith, root crops).
Aerenchyma — has large intercellular spaces, serves to ventilate submerged organs and provide buoyancy.
Water‑storage parenchyma — contains large vacuoles with mucilages that retain water (succulents).
Transfer cells — specialised parenchyma cells with cell wall ingrowths (invaginations) that increase the surface area of the plasmalemma for intensive short‑distance transport of salts and sugars (Raven et al., 2013; Evert, 2006).

Secretory tissues and structures

Main page: Secretory tissues and structures

Synthesise and accumulate or release to the exterior products of metabolism — essential oils, resins, gums, latex, nectar, salts, tannins, alkaloids. They can be of external secretion (glandular hairs, nectaries, hydathodes) or internal secretion (resin ducts, laticifers, cavities, idioblasts with calcium oxalate crystals or tannins). They have crucial protective and signalling functions (Serebryakova et al., 2006; Evert, 2006).

This classification serves as the basis for subsequent in‑depth study of each group of tissues. In the following articles, we will discuss in detail their development, structure, and role in crop formation and plant adaptation to environmental conditions.

4. General Mechanism of Tissue System Functioning

Plant tissues do not exist in isolation. Their work is tightly coordinated in time and space, which transforms the multicellular organism into a single functional system. Let us consider the main principles of this coordination.

Integration of tissues in an organ: the example of a leaf

The leaf is a classic model for demonstrating how different tissue types work together to achieve a common goal — photosynthesis and gas exchange (Raven et al., 2013; Mauseth, 2017).

Epidermis (dermal tissue) performs a barrier function: its transparent cells let light through, while the cuticle and wax protect against excessive water loss. The stomatal apparatus (guard cells and adjacent epidermal cells) regulates gas exchange and transpiration.
Mesophyll (ground tissue) is chlorenchyma, where photosynthesis occurs. In most dicotyledons, it is differentiated into palisade and spongy layers (with loose intercellular spaces), optimising light capture and gas exchange.
Vascular bundles (xylem and phloem) run through the mesophyll as veins. They deliver water and mineral salts to the leaf (via xylem) and export synthesised assimilates (via phloem) to other parts of the plant.
Mechanical tissues (sclerenchyma, collenchyma) often accompany large veins and are located at the leaf margin, providing rigidity and resistance to tearing.

Thus, in a leaf, each tissue type contributes to the overall metabolic process, and their anatomical organisation is subordinated to the single task of maximally efficient photosynthetic performance.

The principle of donor‑acceptor relationships

The life of the whole plant is impossible without the constant redistribution of organic substances. Organs that currently synthesise sugars (mainly mature leaves) are called donors, while those that consume or store them (growing tips, roots, ripening fruits, tubers) are called acceptors (Evert, 2006; Serebryakova et al., 2006).

The connection between donors and acceptors is established through the phloem. The flow of assimilates is directed from the sites of production (sources) to the sites of use or storage (sinks). This flow is not passive: loading of sucrose into the phloem occurs actively, with the involvement of companion cells and specific transporters. A high osmotic pressure is created in the sieve tubes, which drives the solution.

Significance for agronomy: the yield of many crops directly depends on the capacity and efficiency of phloem transport between leaves and storage organs (grain, tubers, fruits). With a lack of potassium, boron, or magnesium, as well as when the phloem is damaged by pests and pathogens, this transport is disrupted, and a potentially high yield is not realised.

Seasonal dynamics: cambial activity, outflow to storage parenchyma, periderm formation

In perennial plants of temperate climates, tissue function follows a distinct annual cycle (Mauseth, 2017; Evert, 2006).

Spring — summer: activation of the cambium (lateral meristem). The cambium begins to divide actively, depositing wide‑lumen elements of early (spring) wood to the inside, ensuring intensive water transport for the opening leaves. Towards the end of the season, narrow‑lumen, thick‑walled fibres of late (summer) wood are formed, providing strength. Similarly, new layers of bast are formed.
Autumn: phloem transport is reoriented from leaves to storage organs — roots, tubers, bulbs. Starch and oils are deposited in storage parenchyma. At the same time, cork cambium (phellogen) is initiated and becomes active in the peripheral tissues, forming a cork layer (periderm) that isolates the old, dying phloem and cortex, and also protects the trunk from winter frosts and desiccation. Leaves are separated from the plant via an abscission layer, leaving a leaf scar covered with cork.
Winter: the cambium and most tissues are in a state of dormancy. Only the living cells of storage parenchyma are active, in which enzymatic processes proceed slowly.

Response to injury

Plants cannot run away from danger, so they have evolved powerful mechanisms for tissue defence at the cellular level. In response to mechanical injury, insect attack, or pathogen infection, several universal reactions are triggered (Evert, 2006; Raven et al., 2013):

Formation of wound meristems and callus. Living parenchyma cells adjacent to the injury dedifferentiate, begin to divide, and form callus — a mass of loose meristematic cells. Callus closes the wound and prevents infection.
Suberisation. Under the influence of hormonal signals (jasmonic acid, ethylene), cells around the injury begin to impregnate their walls with suberin, creating a water‑impermeable and chemically inert barrier. This process is especially important during periderm formation, which separates the affected area from healthy tissues.
Gum exudation (in stone fruits: cherry, plum, apricot). When the wood or cambium is damaged, cells begin to produce a viscous fluid — gum — which fills intercellular spaces and the cavities of dead vessels, mechanically plugging the pathways for pathogens and preventing the spread of rot.
Production of phytoalexins. Cells surrounding the infected zone can synthesise antimicrobial compounds (phytoalexins) that suppress the growth of fungi and bacteria (Serebryakova et al., 2006).

These mechanisms are widely used in agronomy during budding, grafting, and cutting: the successful union of rootstock and scion is possible precisely because of the ability of wound meristems and cambium to form a common callus and restore the continuity of vascular tissues.

5. Factors Affecting the Condition of Tissues

The state and functioning of plant tissues are not fixed once and for all. They constantly change under the influence of a complex of environmental factors, as well as under the influence of biotic agents and mechanical loads. Understanding these relationships is the basis for managing the production process in agronomy (Mauseth, 2017; Evert, 2006).

5.1. Abiotic factors

Light

Light is a key factor determining the development of assimilatory tissues. Under shaded conditions (in the lower forest layer, in dense crops), so‑called “shade leaves” are formed: their mesophyll has only 1–2 layers of loose, poorly differentiated cells, the chloroplasts are large but their number per unit area is small. Under intense illumination (sun leaves), a powerful palisade chlorenchyma develops with several layers of densely packed cells, there are many chloroplasts, and the photosynthetic activity of such tissue is significantly higher (Raven et al., 2013).

The effect of light on mechanical tissues: plants grown under good illumination form more powerful sclerenchyma and collenchyma than etiolated (dark‑grown) shoots. This is directly related to stem lodging resistance.

Water regime

Moisture deficit (drought) causes complex changes in tissues:

Dermal tissues: in xerophytes, a more powerful cuticle forms, often with a waxy bloom, a multilayered epidermis, and stomata are often sunken in pits (crypts) and provided with hairs, which reduces transpiration.
Mechanical tissues: the proportion of sclerenchyma (especially fibres) in stems and leaves increases, collenchyma cells become thicker‑walled. This increases tissue strength under water deficit when turgor pressure drops (Mauseth, 2017).
Vascular tissues: under drought, smaller and thicker‑walled vessels are formed (narrower lumen), which reduces the risk of cavitation (blockage of water columns by air bubbles) at the cost of reduced conductivity. In some species, additional storage parenchyma cells develop in the xylem, serving as water reservoirs.

Excess moisture (especially waterlogging of roots) conversely causes the development of aerenchyma (through programmed cell death) to improve gas exchange under hypoxic conditions.

Mineral nutrition

Nitrogen (N): excess nitrogen (especially the ammonium form) causes vigorous growth of parenchyma tissues (cells become large, thin‑walled). This leads to plant “luxuriance” — a reduced proportion of mechanical tissues, lodging of cereals, and increased susceptibility to fungal diseases (Evert, 2006).
Potassium (K): participates in regulating the turgor of stomatal guard cells and in activating starch‑synthesis enzymes. Under potassium starvation, stomatal function is impaired, and the strength of sclerenchyma fibres (especially in flax and hemp) is reduced.
Silicon (Si): in grasses, the deposition of amorphous silica in the cell walls of the epidermis and mechanical tissues sharply increases their strength and resistance to pests. Silicon fertilisers are used to strengthen straw and increase lodging resistance (Serebryakova et al., 2006).
Calcium (Ca): necessary for the formation of middle lamellae (calcium pectates) and cell walls in general. Calcium deficiency leads to tissue softening, necrosis (blossom‑end rot in tomatoes), and disruption of cambial activity.

Temperature

Extreme temperatures (heat, frost) cause structural changes in tissues:

High temperatures: can destroy chloroplasts, cause denaturation of protoplast proteins, and lead to mass death of meristematic and parenchyma cells.
Low temperatures: cause ice formation in intercellular spaces and, upon rapid freezing, destruction of cell membranes. Winter hardiness of woody species is ensured by the accumulation of protective substances (sugars, proline) in parenchyma cells, as well as by suberisation and lignification of the periderm (Evert, 2006).

5.2. Biotic influences and mechanical loads

Pests and pathogens

Specialised groups of pests and diseases affect specific tissues, which has great diagnostic significance (Serebryakova et al., 2006; Mauseth, 2017):

Vascular wilts (Fusarium, Verticillium, bacterial wilt) — affect the xylem. Pathogens (fungi of the genus Fusarium, Verticillium, bacteria) fill the vessels with mycelium or slime, block them, and release toxins. Visual symptom — wilting of leaves despite moist soil. Diagnosis: on a cross‑section of the stem, darkening of the vascular bundles is visible.
Rusts (fungi Puccinia, Uromyces) — affect the mesophyll of the leaf. The mycelium develops in the chlorenchyma, causing the formation of pustules (uredinia, telia), disruption of photosynthesis, and premature leaf death.
Powdery mildew — superficial mycelium penetrates epidermal cells, causing their death. Stomata become clogged.
Vascular bacteriosis of crucifers (Xanthomonas campestris) — affects the phloem, causing black necrosis and slime formation, leading to wilting and rot.
Root rots — destroy the dermal tissues of the root (rhizodermis, periderm) and the cortex, disrupting water and mineral uptake.

An agronomist’s mistake — confusing symptoms of xylem damage (wilting of the whole plant) and phloem damage (local necrosis, vein swelling, witches’ brooms) — leads to the incorrect choice of fungicide.

Mechanical loads (wind, hail, rain)

Plants in field conditions are constantly subjected to mechanical stress. In response to wind loading (as well as to treatment with retardants), grasses and dicotyledons show increased development of sclerenchyma (especially in nodes and at leaf margins), reduced internode length, and increased stem diameter. This phenomenon is thigmomorphogenesis. On the one hand, it reduces the risk of lodging; on the other hand, it may somewhat reduce total biomass due to additional expenditure of plastic substances on the synthesis of lignin and cellulose (Mauseth, 2017).

5.3. Examples of factor effects on tissues (agronomically significant)

Factor	Typical tissue change	Agronomic consequence
Drought (water deficit)	Cuticle thickening, increased proportion of sclerenchyma, narrowing of vessels	Increased drought tolerance but reduced productivity
Excess nitrogen	Proliferation of thin‑walled parenchyma, reduced mechanical strength	Lodging of cereals, increased disease susceptibility
Application of retardants (chlormequat chloride, mepiquat chloride, tebuconazole)	Thickening of cell walls, enhanced sclerenchyma, shortened internodes	Prevention of lodging, increased stem strength
Boron application	Ensures integrity and functionality of phloem (especially companion cells)	Improves outflow of assimilates to root crops and grain, reduces risk of “hollow grain”
Boron deficiency	Disruption of phloem structure, death of growing points	“Stem cracking” in flax, “heart rot” in sugar beet
Bark damage (rodents, hares, girdling)	Interruption of phloem transport, accumulation of starch and sugars above the damage site	Root death, tree death (especially in fruit trees and young oaks)

Thus, knowing how tissues change under the influence of various factors, the agronomist can purposefully influence plants — through variety selection, fertiliser regimes, irrigation, application of retardants and micronutrients — to produce a crop with the desired properties and resistance to adverse conditions.

6. Plant Histology as a Science

Object and subject

Plant histology is a branch of botany that studies the structure, development (histogenesis), and functioning of plant tissues in close relation to their position in the organ and environmental conditions. Whereas plant anatomy examines the internal structure of organs as a whole, histology focuses on the tissue level of organisation — from cell complexes to their integration into systems (Evert, 2006; Serebryakova et al., 2006).

The subject matter of histology includes:

Morphology of tissues — the shape, size, and mutual arrangement of cells within a tissue.
Histogenesis — the process of tissue formation from meristems, cell differentiation and the acquisition of specialised properties.
Functional specialisation — the relationship between tissue structure and its physiological role.
Plasticity of tissues — changes in tissues under the influence of external and internal factors.
Regenerative capacity — tissue restoration after damage (wound meristems, callus formation).

Histology is closely linked to cytology (cell structure), plant anatomy (organ level), and physiology (functioning), as well as to genetics, molecular biology, and ecology (Raven et al., 2013).

Methods of histological research

A combination of light and electron microscopy, as well as histochemistry and immunohistochemistry, is used to study tissues. The classical scheme includes the following steps (Evert, 2006; Mauseth, 2017; Serebryakova et al., 2006):

Fixation. A freshly cut specimen is quickly treated with fixing solutions (e.g., a mixture of formaldehyde, acetic acid, and ethanol — FAA; or glutaraldehyde — for electron microscopy). This “preserves” the tissues, stopping enzymatic processes and preventing their degradation, keeping the structure as close as possible to the living state.
Dehydration and embedding. The fixed material is dehydrated (passed through alcohols of increasing concentration), then infiltrated with a solvent (xylene or acetone) and embedded in a solid medium — usually paraffin (for light microscopy) or epoxy resin (for electron microscopy). Embedding gives the specimen the hardness necessary for obtaining thin sections.
Microtomy. The embedded block is cut on a microtome (for paraffin blocks) or an ultramicrotome (for resins) into a series of sections of a given thickness:
- For light microscopy — 5–20 µm.
- For electron microscopy — 50–100 nm (ultrathin sections). The sections are transferred to glass slides (light microscopy) or to grids (electron microscopy).
Staining. Most tissues are colourless and lack contrast. To visualise them, specific dyes are used that selectively bind to different components of cell walls and inclusions:
- Histochemistry for lignin (lignification). The most common reagent is a solution of phloroglucinol in hydrochloric acid: lignified cell walls (xylem, sclerenchyma) turn cherry‑red. Alternatives: aniline blue (for lignin), safranin (stains lignified walls red).
- Histochemistry for suberin (suberisation). Sudan III or Sudan black stains suberised (cork) and cutinised walls orange‑red or black. Sudan IV is used to visualise the cuticle.
- Histochemistry for starch. Lugol’s solution (iodine in potassium iodide) stains starch grains blue‑violet and chloroplasts brownish. This allows identification of storage parenchyma and amyloplasts.
- General anatomical staining. The most popular for light microscopy (on permanent slides) is double staining with safranin and astranin (or safranin and fast green): lignified walls become red, cellulose walls green, and the cytoplasm yellow‑green. Another option is toluidine blue, which gives metachromasia: lignin stains blue‑green, pectins pink‑violet.
- Histochemistry for proteins and nucleic acids. For detecting storage proteins in aleurone grains, bromophenol blue or xylidine orange are used. Nuclei and nucleic acids are stained with haematoxylin, methyl green‑pyronin, or fluorochromes (DAPI for DNA, acridine orange for RNA).
Specialised methods:
- Electron microscopy (TEM, SEM). Allows the study of tissue ultrastructure at the level of organelles and cell walls. For example, to reveal plasmodesmata, sieve‑plate pores, the configuration of chloroplast thylakoids, or the structure of periderm boundary layers (Evert, 2006; Raven et al., 2013).
- Immunohistochemistry. The use of labelled antibodies against specific proteins (e.g., against β‑tubulin for microtubules, against xyloglucans for wall matrix) allows these molecules to be localised in tissues with high resolution.
- X‑ray microtomography (micro‑CT). A non‑destructive method that allows 3D reconstruction of the vascular system, the distribution of air spaces (aerenchyma), or the location of mechanical tissues inside an organ without destroying it. It is finding increasing use in phenomics and breeding.
- Laser confocal microscopy. Allows the study of tissues in the living state, stained with fluorescent probes (e.g., for assessing apoplast permeability or the distribution of calcium ions).

Connection with agronomy: histological analysis as a tool

Histological methods give the agronomist a unique opportunity to “see inside” the plant and assess its condition at early stages, long before visible symptoms appear (Mauseth, 2017; Serebryakova et al., 2006).

Breeding for lodging resistance. From a cross‑section of a stem, one can measure the area occupied by sclerenchyma fibres (especially in the sheath of vascular bundles and under the epidermis) and the thickness of cell walls. Varieties with a high proportion of sclerenchyma are less prone to lodging, which is critically important for cereals and industrial crops (flax, hemp).
Early diagnosis of nutrient deficiencies and toxicities.
- Boron deficiency: in growing points and young ovaries, death of meristematic cells and destruction of phloem (companion cells become deformed, sieve plates become blocked with callose) are observed.
- Potassium deficiency: on leaf sections, small brown necroses along the veins are visible, chlorenchyma is destroyed, and epidermal cells shrink.
- Calcium deficiency: middle lamellae fail to form or are destroyed, parenchyma cells lose contact, cavities and cracks appear (e.g., in blossom‑end rot of tomatoes).
- Salinisation: an increase in the number and size of calcium oxalate crystals in vacuoles is revealed, as well as suberisation of root cortex cells (thickening of the exodermis).
Quality assessment of products.
- Flax (spinning quality): the quality of bast fibres (length, diameter, wall thickness, degree of lignification) is directly determined by histological analysis of the stem.
- Cereals (milling properties): the ratio of the aleurone layer (proteins, fats), endosperm (starch), and seed coat (sclereids, fibre) affects flour yield and quality, and baking properties.
- Fruit (shelf life): the condition of the periderm, the intensity of cork formation, and the presence of wound periderm determine the storage life of apples, pears, and citrus fruits.
Disease diagnosis. A histological section of an affected organ is the fastest way to differentiate vascular wilt (xylem clogged with mycelium) from bacterial blight (phloem filled with slime) or phytoplasma infection (companion cells showing abnormal inclusions).
Evaluation of the action of agrochemicals and growth regulators. Under the microscope, one can observe how retardants (e.g., chlormequat chloride) stimulate cell wall thickening in sclerenchyma and reduce the size of parenchyma cells, while herbicides cause membrane destruction and protoplast coagulation in meristems.

Thus, histology is not just a theoretical discipline but a powerful applied tool, allowing the agronomist and breeder to look into the internal structure of the plant and obtain unique information about its condition before problems become externally visible.

7. Significance in Agroecosystems and Practical Management

Understanding the structure and functions of plant tissues is not just a fundamental botanical problem. It underpins modern agronomic management: from variety selection and fertiliser regimes to plant protection and crop quality assessment. In this section, we examine how knowledge of tissues makes it possible to increase the productivity and resilience of agroecosystems.

7.1. Tissues as the basis of productivity and crop quality

Yield — a balance between meristems, assimilation parenchyma and the vascular system

The yield of agricultural crops is determined by how efficiently the plant converts resources (light, water, nutrients) into economically valuable biomass. This process depends on the coordinated work of three key tissue systems (Evert, 2006; Mauseth, 2017):

Meristems ensure the creation of the “factory” — the necessary number of leaves, roots, and generative organs. For example, in cereals, the productivity of the ear is directly related to the activity of intercalary meristems that form a long stem and a large ear.
Assimilation parenchyma (chlorenchyma) of leaves and green stems provides the supply of photoassimilates. The larger the leaf area and the more active the photosynthesis, the higher the potential yield. However, excessive parenchyma growth (e.g., due to nitrogen imbalances) can lead to stem elongation and lodging.
Vascular system (xylem and phloem) must be sufficiently powerful to ensure the upward flow of water and the downward flow of assimilates from leaves to storage organs (tubers, root crops, grains). Narrow vessels or damaged phloem become a “bottleneck” limiting yield.

Balance between these tissue systems is the main target of breeding work: high‑yielding varieties must have not only large leaves but also strong straw (sclerenchyma) and well‑developed vascular bundles.

Quality through tissues: examples for major agronomic crops

Tissue properties directly determine the quality of the final product (Mauseth, 2017; Serebryakova et al., 2006).

Milling properties of grain. A wheat or rye caryopsis consists of three main tissue zones: fruit and seed coats (hard, rich in fibre, almost indigestible), the aleurone layer (one to two rows of thick‑walled parenchyma cells containing proteins, fats, vitamins), and the endosperm (storage parenchyma filled with starch). When milling flour, it is important to separate the seed coat and aleurone layer (bran) from the starchy endosperm. The ratio of these tissues determines the grade of flour and its baking properties.
Spinning quality of flax. In fibre flax, the main product comes from bast fibres (sclerenchyma) located in the stem cortex. The length, thickness, cross‑sectional shape, and degree of lignification of these fibres determine the yield and quality of the yarn. Breeding for a high content of long, thin, elastic fibres is the basis of flax growing.
Shelf life of fruits. Fruits of apples, pears, and citrus lose water and are attacked by pathogens during long‑term storage. Storage stability is ensured by the condition of the periderm: the thicker and higher‑quality the cork layer, the better the fruit is protected. Also important is the ability of storage parenchyma to retain water and the activity of wound meristems, which can seal damaged areas after micro‑injuries.

Role in crop rotation and soil fertility

Plant tissues play a key role in the formation and maintenance of soil fertility (Raven et al., 2013; Evert, 2006).

Dead root tissues as a source of organic matter. The root systems of perennial grasses and green manures (alfalfa, clover, mustard) penetrate the soil, and after dying they leave millions of channels and pores. The cell walls of roots (rich in cellulose and lignin) decompose slowly, turning into humus — a stable soil organic matter. The more powerful the root parenchyma and mechanical tissues of the roots, the greater the contribution to fertility.
Depth of root penetration. It is determined not only genetically but also by the resistance of the root’s vascular and mechanical tissues to soil pressure. Soil compaction (plough pan) hinders the development of root xylem and sclerenchyma, limiting deep penetration. Varieties with stronger root tissues are better adapted to heavy soils.

7.2. Managing tissues through agronomic practices

Knowledge of tissue organisation allows the agronomist to purposefully influence the plant, changing the ratio and properties of tissues in the desired direction (Mauseth, 2017; Evert, 2006; Serebryakova et al., 2006).

Manipulating meristems

Pinching, topping, sucker removal. Removing the shoot apical meristem releases apical dominance and activates lateral meristems (axillary buds). This leads to branching, an increased number of fruit‑bearing branches (tomatoes, cotton), and a redistribution of assimilates from vegetative growth to yield formation.
Treatment with rooting stimulants (auxins, e.g., heteroauxin, indolebutyric acid). Auxin induces dedifferentiation of parenchyma cells in the cutting zone and the formation of adventitious meristems, which then give rise to new root tissues (Evert, 2006). This is widely used in vegetative propagation.

Strengthening mechanical tissues

Retardants (e.g., chlormequat chloride, mepiquat chloride, tebuconazole). These drugs inhibit gibberellin synthesis, which slows the elongation growth of parenchyma cells. As a result, cells of mechanical tissues (collenchyma, sclerenchyma) have time to develop better, their walls thicken, and the proportion of sclerenchyma in the stem increases. The outcome: the stem becomes shorter but thicker and stronger, and lodging resistance in cereals and industrial crops is significantly improved.
Silicon fertilisers. Silicon actively accumulates in the cell walls of the epidermis and mechanical tissues of grasses, forming a silica layer that mechanically strengthens the walls and hinders pest feeding. Silicon fertilisation (e.g., with diatomite or liquid silicon) is effective in reducing lodging and pest damage in rice, wheat, and oats.

Optimising the vascular system

Foliar boron fertilisation. Boron (B) is a critically important micronutrient for the formation and functioning of the phloem, especially for companion cells, which support the life of sieve tubes. Under boron deficiency, the phloem breaks down, and the outflow of assimilates from leaves to roots and storage organs ceases. Foliar boron applications at the bud‑burst and flowering stages improve sugar transport to ovaries and root crops, preventing “hollow grain” and “heart rot” in sugar beet (Evert, 2006).

Controlling the development of dermal tissues

Use of adjuvants with wax‑preserving effects. The efficacy of pesticides depends largely on how well the droplet wets the epidermal cuticle. Adjuvants reduce surface tension, and some (e.g., those based on vegetable oils) even partially dissolve waxy layers, improving penetration.
Consideration of cuticle stage when applying pesticides. Young leaves still have a thin, highly permeable cuticle, but old leaves have a thickened cuticle, making pesticide penetration more difficult. Spraying at early growth stages may be more effective.

7.3. Diagnosis and assessment of tissue condition

Visual signs at the organ level

An experienced agronomist can infer much about tissue condition simply by examining the plant:

Interveinal chlorosis — a typical symptom of a problem in the mesophyll (assimilation parenchyma). Often associated with magnesium (Mg) deficiency — an element of chlorophyll — or iron (Fe) deficiency, which is necessary for chlorophyll synthesis. The veins remain green because the tissues around them (vascular system and adjacent parenchyma) receive more of these elements (Raven et al., 2013).
Cracking of bark (e.g., in apple, birch) — indicates a disturbance in the function of the cambium and phellogen. Causes can vary: rapid growth (cambium cannot produce enough cells), frost damage, fungal diseases (Nectria canker), or mechanical injury.

Simple field microscopy

Even a small hand microscope (20–50× magnification) can provide valuable information:

Cross‑section of a cereal stem to assess lodging. If you make a cross‑section of the stem and examine it under a hand lens, you can assess:
Proportion of sclerenchyma — how much space is occupied by thick‑walled cells in the sheath of vascular bundles and under the epidermis. The more sclerenchyma, the higher the lodging resistance.
Thickness of cell walls (approximately, by optical density). Resistant varieties have thicker and more intensely lignified walls.
This helps the breeder discard weak lines in the field.
Cross‑section of a beet or carrot root crop — the thickness of storage parenchyma and the presence of intercellular spaces can be assessed. This indirectly indicates juiciness and shelf life.

Typical mistakes: confusion between xylem and phloem damage symptoms

One of the most common and costly errors in agronomy is misdiagnosis of vascular tissue diseases (Mauseth, 2017; Evert, 2006).

Symptom	Affected tissue	Typical diseases	Treatment error
Wilting of the whole plant (starting with lower leaves, then whole), vascular bundles on stem or root section turn brown, clogged with mycelium or slime	Xylem (vessels)	Fusarium wilt, Verticillium wilt, bacterial wilt	Use of contact fungicides that do not penetrate the xylem (systemic, xylem‑mobile fungicides — e.g., benzimidazoles — are required)
Local necrosis, vein swellings on leaves, witches’ brooms, yellowing and curling of leaves without systemic wilting, gum exudation from phloem areas	Phloem (bast, sieve tubes)	Phytoplasma diseases (e.g., stolbur, witches’ broom), viral infections, phloem bacteriosis (e.g., Candidatus Liberibacter — citrus huanglongbing)	Treatment with antifungal agents (ineffective against phytoplasmas or viruses). Antibiotics (bactericides) or removal of infected plants are needed.

Rule of thumb: If the plant is wilting and a stem section shows brown, clogged xylem — it is a vascular disease (xylem). If there is no wilting but there are deformations and vein necrosis — the phloem is likely affected. Accordingly, the choice of product and treatment strategy will be different.

Thus, managing plant tissues is not an abstract theory but the everyday practice of an agronomist, starting from variety selection and ending with product quality assessment. A deep understanding of tissue organisation is the key to high and stable yields.

References

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